When population estimates derived at 7,000, 8,000, and 9,000 cfs are 

 plotted against discharge, the following regression equation results (figure 60) 



log population = 4.9384 + 0.000042 discharge (cfs) 



This equation permits a prediction of population of invertebrates at any dis- 

 charge. One should remember that a regression equation is a mathematical 

 tool that may or may not predict a future biological event. Population 

 estimates may continue decreasing linearly as the regression equation 

 indicates. In this case the regression line is probably roughly accurate. 

 Because of the channel morphology in the Intake area, decreases in discharge 

 result in decreasing currents across the entire channel, and little bottom 

 habitat is exposed in the process. However, at some low discharge, large 

 amounts of river bottom would be exposed with resultant loss of habitat and 

 a dramatic decrease in fauna. The effects of reduced current velocity and 

 of loss of bottom habitat are separable in their effect on fauna. Reduced 

 current velocities (due to lowered streamflow) could adversely affect 

 bottom fauna even before a significant loss in bottom habitat occurred. 



Using the regression equation (figure 60), population estimates in a 

 one-meter-wide strip at Intake can be calculated for lower discharges: 



6000 cfs 156,000 organisms 



5000 cfs 141,000 organisms 



4000 cfs 128,000 organisms 



3000 cfs 116,000 organisms 



2000 cfs 105,000 organisms 



1000 cfs 96,000 organisms 



These estimates, based on data gathered in November, are higher than estimates 

 would be based on data gathered later in the winter or in the spring, because 

 of natural mortality and drift out of the study area. 



As flows decrease, other factors--ice and silt--would undoubtedly result 

 in a higher-than-normal mortality of invertebrates. With decreased discharges, 

 ice cover would tend to be thicker than normal, thus freezing larger-than- 

 normal areas of river bottom and resulting in a greater amount of molar action 

 during spring ice break up. Low discharges and reduced currents during the 

 spring would permit greater amounts of silt to accumulate, resulting in a 

 detrimental effect to bottom-dwelling organisms. 



Evidence confirming the "stream continuum" theory is apparent, although 

 not in large quantities. One major problem with implementing this theory 

 in the west involves stream order. With the multitude of tributaries to 

 every stream a large creek might be of order 10 to 15 by the time it 

 reaches a larger river. The Yellowstone River could conceivably be of order 

 20 or more, although this has never been calculated. Some of the basic tenets 

 of the theory are evident. The invertebrate fauna in stations 1-8 is 

 dominated by shredder-type organisms. The fauna in the middle and lower 

 river is dominated by collector organisms, e.g., the Trichoptera family 

 Hydropsychidae, which build small nets to collect small food particles and 



100 



